Heating rate effect of thermal expansion in granite and implications for rock breaking

Abstract

The influence of the heating rate on the thermo-mechanical response and damage evolution of rock is a critical factor limiting the safety and efficiency of engineering applications. Conventional models are limited, however, as they assume a static coefficient of thermal expansion (CTE) and ignore its dynamic nature under rapid thermal loading. This study confronts this knowledge gap using a synergistic experimental–numerical approach. A custom system combining induction heating and Digital Image Correlation was employed to measure the rate-dependent CTE of both bulk granite and its constituent minerals over various heating rates. These dynamic coefficients were then integrated into a high-fidelity numerical model to simulate microwave-assisted rock breaking. Results definitively show the CTE is strongly rate-dependent. While the quartz phase transition at ∼573 °C triggers critical damage, faster heating significantly amplifies strain localization and damage accumulation. Crucially, simulations revealed that under identical microwave loading, the model using dynamic CTE (530 °C/min) reached a 1000 mm2 failure area 11 times faster than the model using quasi-static CTE (5 °C/min). This study fundamentally establishes rock's CTE as a dynamic, rate-dependent property, providing a key scientific basis for advancing such thermal fracturing technologies.

1. Introduction

A comprehensive understanding of the mechanical response of rocks under complex thermo-mechanical coupling conditions constitutes the foundation for ensuring the safety and efficiency of numerous deep geological engineering projects, and represents one of the core frontiers in Earth sciences [1]. The performance of rock masses subjected to significant temperature variations is a decisive factor in the success and risk profile of applications such as geothermal energy exploitation aimed at increasing the share of renewable energy, deep geological disposal of high-level radioactive waste for long-term environmental security [2], deep mineral resource extraction, underground energy storage, and fire safety design and post-disaster assessment of critical infrastructure including transportation tunnels [3,4].

Across these diverse scenarios, rocks are inevitably exposed to thermal loading or thermal shock at varying rates. Such thermal disturbances induce expansion and contraction of the material, alter the pre-existing stress field within the rock mass, and can ultimately trigger the initiation, propagation, and coalescence of thermally induced microcracks—a process referred to as thermal damage [5]. The progressive accumulation of thermal damage irreversibly degrades the macroscopic mechanical properties (e.g., strength and stiffness) and transport characteristics (e.g., permeability) of the rock, thereby directly affecting the permeability and productivity of geothermal reservoirs [6,7], the integrity of host rock barriers for nuclear waste repositories [8], and the long-term stability of underground engineering structures [9]. In these critical applications, the primary concern is often not catastrophic macroscopic failure, but rather the sub-critical degradation of the rock mass's mechanical integrity and hydraulic properties. Even a minor increase in fracture-induced permeability can compromise the efficiency of a geothermal system or the safety of a nuclear waste repository. Therefore, understanding and predicting the entire process of thermal damage, from the first microcrack onwards, is of paramount importance.